1. Field of the Invention
The present invention relates to a method of filling a capsule having a preferably ring-shaped or disc-shaped cavity with powder which, by a subsequent isostatic pressing, is transformed into a solid body with a high density. The invention also contemplates a centrifugal filling machine for the capsule for carrying out the method.
2. Prior Art
Rings or discs for turbines, especially for gas turbines, comprise alloys with a high percentage of additives of alloying materials, so-called superalloys, which provide a material with good strength properties at high temperatures. Such superalloys are difficult to machine, both during casting and during the subsequent forging and machining operations. When manufacturing blanks an uneven composition is obtained through segregation of alloying materials during solidification. The subsequent forging operation has to be carried out within a temperature interval often requiring close tolerances. This forging may be rendered difficult by material segregation. The surface layers may absorb gases which deteriorate the properties of the material. This may necessitate considerable working allowance, i.e. a great deal of material has to be removed by expensive machining. In addition, the material in itself is expensive which means that a great deal of material has to be scrapped during machining, and this contributes to increase the cost of production of the turbine discs.
For the above reasons, production of turbine discs has recently evolved, in which the starting material is a powder which is filled into capsules having substantially the same shape as the turbine discs when finished, and which is bonded together by hot isostatic pressing to form a dense homogeneous body. The dimensions of the capsule must be determined with regard to the degree of filling, shrinkage and distortion, dependent on the degree of filling during the hot pressing. The degree of filling is dependent on the distribution of the grain size in the powder and the distortion is dependent on how even the density is in the powder of the filled capsule. A suitable distribution of grain sizes from the point of view of filling, with fine fractions which fill up the gaps between coarse fraction in a satisfactory manner, may provide a very good degree of filling under favorable filling conditions; however, under unfavorable filling conditions, a separation of coarse and fine grain fractions may be obtained. This separation results in a deteriorated degree of filling and increased shrinkage and distortion, for example skewness, because the density is different in different parts of the powder in a capsule.
Heretofore, rotary-symmetrical capsules made into, for example, turbine discs, have either been filled from the periphery in a stationary capsule, or from the center in a rapidly rotating capsule. During the fall down to the capsule bottom, or to powder already located there, an unfavorable separation has occurred. When filling a rotating capsule, the capsule rotates so rapidly that the powder is thrown with great force outwards towards the periphery. However, it is also known that when filling during rotation a considerable and harmful separation of coarse and fine powder grains may be obtained. Contrary to expectation, the powder is not deposited evenly in the capsule. On the contrary, there is obtained an uneven structure of powder so that a wave shape is obtained with respect to a circle concentric with the capsule center. From inwardly-directed wave crests coarser powder grains roll down into the wave trough, where a concentration of coarse grains is obtained, thus obtaining an uneven density. This results in uneven shrinkage and distortion, which necessitates an undesirable excessive measure of the capsule.